24 Biological Chemistry. 



gently at first, but the temperature is afterwards raised, 

 and the mixture is warmed until it is colourless ; water is 

 then added, and the mixture is boiled for a few minutes 

 until nitrous fumes cease to be evolved. A few drops of 

 fifty per cent, ammonium nitrate solution are then added 

 to the cooled solution, and then a few drops of ten per 

 cent, ammonium molybdate solution. In the event of the 

 presence of phosphorus in the substance a yellow preci- 

 pitate of ammonium phosphomolybdate will form, especially 

 if the mixture is gently warmed. 



Metals. Attention has already been called (p. 10) to 

 the fact that certain plant and animal products contain 

 metals in a state of chemical combination. Such substances 

 on ignition are not entirely destroyed, but an " ash " is 

 always left, which may be investigated by the ordinary 

 methods of qualitative inorganic analysis. 



Quantitative Organic Analysis. 



To Lavoisier must be ascribed the merit of having 

 recognized that organic substances on burning give rise to 

 carbon dioxide and water, the carbon being, on complete 

 oxidation, converted into the former substance, and the 

 hydrogen into the latter; by estimating the amounts of 

 these products formed from a given weight of a substance, 

 the amount of carbon and hydrogen can be calculated. 

 The technique as employed by Lavoisier was, however, 

 cumbersome, and it was not until Liebig, in 1830, pub- 

 lished his methods of analysis that the composition of 

 organic compounds could be determined as an ordinary 

 routine process of research. These methods, with certain 

 not very essential modifications of detail, have remained in 

 vogue up to the present day ; and the method described 

 below, which is due to Dennstedt, is founded on the process 



